As a method of forming a thin film on the surface of a substrate such as glass, silicon wafer, and the like, it is conventionally known to use a sputtering apparatus. In this sputtering apparatus, for example, a predetermined sputtering gas (argon gas) is introduced into a processing chamber in vacuum atmosphere, and a negative DC voltage is applied by a power supply apparatus to a target which is manufactured depending on the composition of the thin film to be formed on the surface of the substrate, thereby forming a plasma atmosphere. Then, ions in the plasma atmosphere are accelerated toward, and collide against, the target to thereby cause the sputtered particles (target atoms) to scatter and to get adhered to the surface of the substrate for deposition thereon, whereby a predetermined thin film is formed.
It is known that arc discharge (anomalous electric discharge) occurs for one reason or another during thin film forming by the above-mentioned sputtering apparatus. When arc discharge occurs, the impedance of the plasma (load) becomes rapidly low. Therefore, a rapid voltage drop occurs, accompanied by a consequent increase in current. Here, in case the target is of metallic make, especially of aluminum and the like, when arc discharge of high arc current value locally occurs on the target, the particles or splashes (blocks or masses of several μm to several hundreds μm) to be generated by melting and emission of the target will occur, thereby giving rise to a disadvantage in that good film forming cannot be made.
It is normal practice for the power supply apparatus, which applies DC voltage to the above-mentioned target, to be provided with an are processing unit. As this kind of axe processing unit, mention may be made of one having: detecting means for detecting an output voltage or an output current from a DC power supply unit to the target; and those switching elements (arc switches) for cutting off the arc discharge which are disposed in parallel or in series between the positive and the negative outputs from the DC power supply unit to the target. Then, after having detected the occurrence of arc discharge by the amount of changes in the output voltage or output current, the above-mentioned switching elements are either short-circuited in case of parallel connection or are opened in case of series connection to thereby shut off the supply of arc energy to the plasma (load).
Further, in order to solve the above-mentioned disadvantage, there is also known a method in which the output impedance of the power supply apparatus is made to be of constant current. Ordinarily, there are many cases in which capacitors are disposed in parallel with each other in the output from the power supply apparatus to the target. The characteristics per unit time when the supply of the arc energy is shut off are shown to be of constant voltage. Then, when arc discharge occurs in the plasma, overcurrent rapidly occurs at a short period of time due to constant-voltage characteristics, resulting in an increase in the arc energy. However, by operating the above-mentioned switching elements at the time when the occurrence of the arc discharge is detected, the overcurrent can be prevented from increasing. But during the time interval of several μS in which the switching elements are actuated, this rapid increase in the current cannot be avoided.
Now, therefore, in order to prevent this rapid increase in the current, there is known in Patent Document 1 a method in which the output characteristics of the power supply apparatus are changed by switching the arc switches to constant current during the time required for the processing of suppressing the arc discharge (arc discharge suppression processing) so that the increase in the arc current can be prevented to the best extent possible during the time until the switching elements are actuated. In the method as described in Patent Document 1, however, there is a big problem in the characteristics after the arc processing has been finished. In other words, according to the method as described in Patent Document 1, when the output is brought back to normal after having finished the control of the switching elements thereby finishing the arc processing, there is a problem in that overvoltage occurs due to the inductance components of the plasma (load). This kind of problem, becomes remarkable when the area of the target, as a cathode, is large and the volume of the plasma becomes large (for example, when the external dimensions of a target become the order of several meters in a sputtering apparatus for manufacturing large-area flat panels, the transient inductance components will be more than several tens μH).
Then, due to a rapid increase in current from the output of the DC power supply apparatus which has been made to be constant current due to the inductance component, the voltage in the plasma will be more excessive than ordinary discharge voltage. For example, when the ordinary discharge voltage is −600 V, a voltage above −1000 V will be generated after having finished the arc processing by the switching elements in the arc processing circuit if the DC power supply apparatus is made to be of constant current. If this kind of overvoltage occurs, arc discharge comes to frequently occur.